Various forms of fastening structure with screws having a function preventing loosening are known conventionally. A common one is by a fastening method called a “double nut”. This fastening method is one in which a nut 1 of an internal thread is fastened on to a threaded portion of a dual-threaded screw structure (of an external thread) and then a nut 2 of an internal thread is fastened to come in contact with the nut 1 so that tensional force (axial force) acts between both nuts. By mutual fastening with the nut 1 and nut 2, loosening of the screw is prevented from occurring by the vibration, etc., of a structure where this double but is used. In this “double nut”, the nut 1 is one for preventing loosening (a thinner nut) and the nut 2 fastening from backward is one for fastening (a thicker nut).
A technique improving this is proposed in which threads having different pitches (e.g., a coarse thread and a fine thread) are formed on a threaded portion (of an external thread) and fastening by a nut for a coarse thread and a nut for a fine thread is made, so that the difference of pitch in both threads provides a function preventing loosening. As a method for manufacturing bolts used in this, a method for manufacturing a multi-threaded screw bolt is known in which a coarse thread and a fine thread are formed thereon (see, e.g., Patent Document 1). Further, a technique regarding a bolt preventing loosening is also known in which either one of a coarse thread and a fine thread is a multi-threaded thread (see, e.g., Patent Documents 2 and 3).
Furthermore, a technique regarding a shape of a thread of a bolt manufactured by rolling is proposed in which the thread of the bolt is composed of a coarse thread having a first standard pitch and a fine thread having a second pitch smaller than the first pitch, in order to equalize load of rolling as plastic deformation in rolling (see Patent Document 4).